POMGnT1, a protein involved in muscle-eyebrain disease, is located in the Golgi complex in the mouse retina and 661W photoreceptors

Abstract

The POMGNT1 gene, encoding protein O-linked mannose β1,2-Nacetylglucosaminyltransferase 1, is associated with muscle-eye-brain disease (MEB) and other dystroglycanopathies. Its lack of function or expression causes hypoglycosylation of alpha-dystroglycan (α-DG) in muscle and CNS (including brain and retina), where it catalyzes the transfer of N-acetyl-glucosamine to O-mannosylated α-DG. MEB patients ocular symptoms include retinal dysplasia and detachment, glaucoma and abnormal electroretinogram. In this context, α-DG O-glycosylation is known to be relevant for the establishment of functional synapses between photoreceptors and their postsynaptic, bipolar cells in the retina. Nonetheless, the POMGnT1 expression pattern in the normal mammalian retina remains to be addressed. In this work we have demonstrated by means of RT-PCR and immunoblotting analyses that the POMGNT1 gene is expressed at the mRNA and protein levels in all mammalian species studied, from rodents to humans. Immunohistochemical studies have shown that it concentrates in the myoid portion of mouse photoreceptor inner segments, where it colocalizes with GM130, a Golgi complex protein marker. Its presence in the Golgi has also been revealed in 661W cells, a mouse cone photoreceptor immortalized cell line. However, and in contrast to retinal tissue, POMGnT1 additionally accumulates in the nucleus in 661W photoreceptors, where it colocalizes with POMT1 and POMT2, two proteins known to form a heterodimer active in the O-linked addition of mannose to (unglycosylated) α-DG. These results are suggestive that POMGnT1 not only participates in the O-glycosylation of α-DG in the Golgi complex, but also of other yet-to-be-identifi ed proteins in the nucleus of mouse photoreceptors.